A hearing impaired person typically suffers from a loss of hearing sensitivity that is frequency dependent and dependent upon the sound level. Thus, a hearing impaired person may be able to hear certain frequencies (e.g., low frequencies) as well as a non-hearing impaired person, but unable to hear sounds with the same sensitivity as the non-hearing impaired person at other frequencies (e.g. high frequencies). Similarly, the hearing impaired person may be able to hear loud sounds as well as the non-hearing impaired person, but unable to hear soft sounds with the same sensitivity as the non-hearing impaired person. Thus, in the latter situation, the hearing impaired person suffers from a loss of dynamic range.
With respect to dynamic range loss, typically a compressor is used to compress the dynamic range of the input sound so that it more closely matches the dynamic range of the intended user. The slope of the input-output compressor transfer function (ΔI/ΔO) is referred to as the compression ratio. Generally the compression ratio required by a user is not constant over the entire input power range.